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Autor Wątek: testowanie MOSfeta  (Przeczytany 6761 razy)
markcomp
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« : Czerwiec 05, 2010, 11:11:00 »

testowanie MOSfeta




MOSFET testing
http://www.4qdtec.com/mostest.html

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A Simple test

Properly testing a MOSFET requires a lot of expensive test gear, but if you have a suitable digital multimeter you can do a pretty accurate go/no go test which will fail very nearly all dud MOSFETs.

Nowadays most multimeters have a diode test range. On most multimeters (but by no means all!) this puts about 3-4v across the device under test. This is enough to turn on most MOSFETs - at least partially, and enough to test. The meters which use a lower open-circuit test voltage (sometimes 1.5v) will not make this test!



So: connect the meter negative to the MOSFET's source. This is arrowed in the drawing above which shows the most popular TO220 MOSFETs.

Hold the MOSFET by the case or the tab if you wish, it doesn't matter if you touch the metal body but be careful not to touch the leads until you need to.

First touch the meter positive on to the gate.

Now move the positive meter probe to the drain. You should get a low reading. The MOSFET's gate capacitance has been charged up by the meter and the device is turned on.

With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you wish, it matters not). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conducting device.

Such a simple test cannot be 100% - but it is useful and is usually adequate.




How to test whether MOSFET has gone bad or not?
http://www.edaboard.com/ftopic279152.html

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With a multimeter on diode test setting.
First test whether there is any short between Drain-Source, this should indicate an open circuit unless there is an internal protection diode which usually measures 0.6V.
Second, test all ther combinations for shorts, e.g. Drain-Gate, Gate-Source.

In case of a good mosfet, a voltage at the gate should produce a short (continuity actually) between Drain and Source. By touching the gate ao as to discharch or by connecting it to the Source pin so as to discharge it, the continuity between Drain-Source should disappear and you should measure nothing, open circuit. If the continuity remains then the mosfet is bad.
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« Odpowiedz #1 : Czerwiec 05, 2010, 11:15:02 »

Testing a MosFet
http://www.utm.edu/staff/leeb/mostest.htm



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This testing procedure is for use with a digital multimeter in the diode test-range with a minimum of 3.3 volt over d.u.t. (diode-under-test)



Connect the 'Source' of the MosFet to the meter's negative (-) lead.

1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

2) First, touch the meter positive lead onto the MosFet's 'Gate'.

3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.

Such a simple test is not 100% -- but is useful and usually adequate.

When MOSFETS fail they often go short-circuit drain-to-gate. This can put the drain voltage back onto the gate where of course it feeds (via the gate resistors) into the drive circuitry, possibly blowing that section. It will also get to any other paralleled MosFet gates, blowing them also.
So, if the MosFets are deceased, check the drivers as well! This fact is probably the best reason for adding a source-gate zener diode; zeners fail short circuit and a properly connected zener can limit the damage in a failure! You can also add subminiature gate resistors -- which tend to fail open-circuit (like a fuse) under this overload, disconnecting the dud MosFet's gate.
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« Odpowiedz #2 : Sierpień 04, 2011, 11:41:05 »

Basic Testing of Semiconductor Devices  and other information including  Introduction to Curve Tracers

http://www.repairfaq.org/sam/semitest.htm


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Testing MOSFETs
 (From: Paul Mathews (optoeng@whidbey.com).)
Verify gate has infinite resistance to both drain and source. Exception: FETs with protection circuitry may act like there is a zener shunting GS, i.e., diode drop for gate reverse bias, ~20V breakdown in fwd bias.

Connect gate to source. Drain to source should act like a diode.

Forward bias GS with ~5 V. DS in forward bias should measure very low ohms. In reverse bias, it will still act like a diode.
 The usual failure mode: GS short AND DS short. In other words, everything connected together.

 (From: Richard Torrens (4qd@argonet.co.uk).)

 A lot of common multimeters have a diode range: you can use this to measure a MOSFET out of circuit and get a good idea of whether it is OK. Meter negative on the source, you should get no reading (open circuit) on the drain. Not on the gate but if you measure the drain AFTER measuring the gate you will find it conducts. A finger between source and gate will bleed away the charge and the MOSFET stops conducting.

 You really need a 'scope to check the drive circuit. What it does will depend on the circuit configuration, whether there is current limiting etc.

 (From: E. Wolsner (interser@algonet.se).)

 My way of testing a power MOSFET is indeed simple and normally sufficient:

 One ohmmeter is connected to the drain and the source, measuring the resistance between drain and source, which should be very high. Another ohmmeter is connected between gate and source. This ohm-meter should have a high resistance capability (maybe 20 M ohms) and thus have a relatively high test voltage (more than 5 volts). Now this voltage, when connected with the proper polarity, will turn the mosfet on, which will be indicated by the first ohm-meter. It will show zero resistance. To turn the transistor off, you reverse the gate-source voltage, and the drain-source ohm-meter will again indicate high resistance.

 (Portions from: Egon Wolsner (interser@algonet.se).)

 The multimeter must be able to provide at least 5 volts output on the resistance measuring range (this usually means that a DMM will not work). If it does, here is the procedure:

 First you measure the resistance between the drain and source terminals, it should be infinity. Then connect the plus to the gate and the minus to the source pin. That should turn the MOSFET on. Then you measure the resistance between the drain and source pins, which should verify that the resistance is indeed near zero. (The gate capacitance will hold the device in the on-state long enough for this test.) Turn the mosfet off by shorting the gate and source pins (for a n-channel MOSFET)

 (From: Bruce (reglarnavy@aol.com).)

 You can get a pretty good idea about the condition of a MOSFET with some quick & simple bench tests. The first thing you can do with a meter is measure the parasitic substrate diode that connects the drain to the source. In an NMOS part, this diode's cathode will be at the drain, and the anode at the source. It will meter out similar to any conventional diode in both fwd / reverse directions. You can see this diode in the schematic representation of the FET in some databooks and a few schematics. The FET should show infinite resistance, gate - source and gate - drain. If it does not, then the gate oxide may be blown.

 A second simple test can be done with a meter and a 9 V battery. First, short the gate to the source to discharge any stored charge there. Then put your meter on ohms and connect it across the drain - source. It should measure as an open. Briefly connect the 9 V across the gate (+) to source (-) , again, NMOS polarities, and the meter resistance should fall to a very low resistance, on the order of an ohm or less. Removing the battery will not change the reading, because in a good FET, Ciss will remain charged for a long time and keep the FET on. Most FETs come on at Vgs=2 volts or so.

 If these two tests work, then the FET is off to a good start. Substituting a power supply and a proper load resistor for the meter, and a variable voltage (a pot across the 9v will work) for the Vgs supply, in the aforementioned test, will obviously be a more realistic test, and will also let you measure Vds, Id, etc.

 BEWARE ESD WITH FETS! Wear a wrist strap, keep the parts away from insulators like plastics, and make sure your soldering iron tips are grounded. If you do not have any of the black ESD foam to keep your parts in, then look around for an anti-static bag that once may have contained a computer board, SIMMs, etc.




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Testing IGBT
 Basic testing IGBTs (Insulated Gate Bipolar Transistors) should be similar to an enhancement mode N-channel MOSFET except that the threshold voltage may be larger than a typical MOSFET (e.g., 8 V instead of 4 V).

-Check for shorts with an ohmmeter.
-Put a current limited supply across C-E with an ammeter in series.
-Apply at least 10 V G-E. The transistor should turn on.
-Ground the gate. The transistor should turn off.

 A curve tracer should be able to be configured for IGBT testing to determine more complete behavior.
« Ostatnia zmiana: Sierpień 04, 2011, 11:44:04 wysłane przez markcomp » Zapisane

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« Odpowiedz #3 : Sierpień 04, 2011, 11:46:21 »

http://www.firstwatt.com/pdf/art_mos_test.pdf

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